Hemp Fiber Reinforced Composite with Recycled High Density Polyethylene and Production Thereof

ABSTRACT

Novel structural materials composed of industrial hemp fiber with recycled high density polyethylene (HDPE) as well as methods for the production of the same are disclosed. The material&#39;s mechanical strength outperforms that of conventional lumber and could compete with glass fiber reinforced composites, particularly in tensile strength. In addition, this material offers many other significant advantages including insect free, high moisture resistance, no harmful chemical treatments, and no rapid corrosion in water environments.

RELATED APPLICATIONS

This U.S. non-provisional patent application claims priority under 35U.S.C. §119 to pending U.S. non-provisional patent application No.14/279,429 filed, on May 16, 2014, which claims priority to U.S. Pat.No. 9,187,624 issued on Nov. 17, 2015, and to U.S. Provisional PatentApplication No. 61,375,332 filed on Aug. 20, 2010, and to U.S.Provisional Patent Application No. 61/386,706 filed on Sep. 27, 2010.

GOVERNMENT SUPPORT

This invention was made with Government support under award number0548401 by the National Science Foundation. The Government has certainrights in the invention.

BACKGROUND

The use of high density polyethylene (HDPE), continues to grow in theUnited States and abroad. HDPE is used for the production of consumerarticles including, but not limited to, liquid containers such as juice,milk and water bottles, laundry detergent bottles, and toys, forexample. Milk bottles and other hollow goods manufactured through blowmolding are the most important application area for HDPE—More than 8million tons, or nearly one third of worldwide production, was appliedhere. Above all, China, where beverage bottles made from HDPE were firstimported in 2005, is a growing market for rigid HDPE packaging, as aresult of its improving standard of living. In India and other highlypopulated, emerging nations, infrastructure expansion includes thedeployment of pipes and cable insulation made from HDPE. The materialhas benefited from discussions about possible health and environmentalproblems caused by PVC and Polycarbonate associated Bisphenol A, as wellas, its advantages over glass, metal and cardboard One third of all toysare manufactured from this thermoplast. Less often seen, butnevertheless vital goods produced from HDPE include water pipes, gasmains, oil tanks, and geomembranes. The lightweight, non-toxic materialis easily recyclable and is increasingly being utilized as analternative for less environmentally friendly substances.

Another application for HDPE is wood plastic composites, composite woodand plastic building materials to replace wood, concrete and metalcomponents. Recycled plastics may be used to produce these materials.HDPE is also widely used in the pyrotechnics trade. HDPE mortars arepreferred to steel or PVC tubes because they are more durable and moreimportantly they are much safer compared to steel or PVC. If a shell orsalute were to malfunction (flowerpot) in the mortar, HDPE tends to ripand tear instead of shattering into sharp pieces which can kill or maimonlookers. PVC and steel are particularly prone to this and their use isavoided where possible.

Recently, the global HDPE market for HDPE reached a volume of more than30 million tons, up from 22 million tons in the year 2000. There istherefore a strong need in the market for new uses of the growingamounts of used HDPE and new materials made from recycled HDPE. There isalso a strong need in for new building materials with increased strengthand resistance to water and insect damage.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to a novel structural materialcomprising hemp fiber with (HDPE) as well as methods for the large scaleproduction of the same. The HDPE may be recycled HDPE. The compositematerial's mechanical strength outperforms that of conventional lumberand could compete with glass fiber reinforced composites, particularlyin tensile strength. In addition, this material offers many othersignificant advantages including insect free, high moisture resistance,no harmful chemical treatments, and no rapid corrosion in waterenvironments.

Embodiments of hemp fiber composites with recycled high densitypolyethylene matrix were prepared in various compositions ranging from20 to 40% of fiber volume fraction. The fiber-matrix interface wasimproved by treating the hemp fiber with NaOH prior to producing thecomposite. The hemp fiber-recycled high density polyethylene (rHDPE)composites achieved maximum tensile strengths on the order of 60 MPa.Among the tested samples, the composites with 40% of fiber volumefraction demonstrated the best mechanical properties with regards totensile strength, elastic modulus, and flexural strength and modulus.

Hemp fiber composites with rHDPE were manufactured by using extrusion,pultrusion, vacuum assisted infusion, and compression molding processtechniques. Prior to composite fabrication, the natural hemp fibers weretreated with a NaOH solution. The effect of alkali treatment wasinvestigated by FTIR and SEM. FTIR results indicated that there is anincrease in the percentage of —OH groups, which may provide morereaction sites for fiber-matrix adhesion. Therefore, the interfacialadhesion between the fiber-matrix may possibly be increased. Pectin, waxand lignin were completely removed from hemp fiber surface, whichresulted in huge surface area and improved surface roughness. The FTIRalso indicated that the hemicelluloses group was partially removed. SEMimages of treated hemp fiber support the conclusions from FTIR results.SEM images of fracture surface of hemp fiber composites showed clearlyimproved interfacial adhesion between the hemp fiber and polymer matrix.The resultant composites have demonstrated promising mechanicalproperties with regard to their tensile strength, tensile modulus andstrain at maximum strength, flexural strength and modulus for each hempfiber composite which has been studied. Based on these reportedexperimental results, the hemp fiber-rHDPE composites with 40% fibervolume fraction yielded very promising results of tensile strength andmodulus, and flexural strength and modulus of 60.2 MPa, 2575 MPas 44.6MPa and 2429 MPa (at 1% strain) respectively. The resultant compositehave a good potential for light load applications in civilinfrastructure industry, for instance short-span bridges and hurricaneproof panels.

The terminology used herein is for the purpose of describing, particularembodiments only and is not intended to be limiting of the invention. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items. As used herein, the singularforms “a,” “an,” and “the” are intended to include the plural forms aswell as the singular forms, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, steps, operations, elements, and/orcomponents, but do not preclude the presence or addition of one or moreother features, steps, operations, elements, components, and/or groupsthereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by onehaving ordinary skill in the art to which this invention belongs. Itwill be further understood that terms, such as those defined in commonlyused dictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art and thepresent disclosure and will not be interpreted in an idealized or overlyformal sense unless expressly so defined herein.

In describing the invention, it will be understood that a number oftechniques and steps are disclosed. Each of these has individual benefitand each can also be used in conjunction with one or more, or in somecases all, of the other disclosed techniques. Accordingly, for the sakeof clarity, this description will refrain from repeating every possiblecombination of the individual steps in an unnecessary fashion.Nevertheless, the specification and claims should be read with theunderstanding that such combinations are entirely within the scope ofthe invention and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph of a testing sample of the composite materialcomprising hemp fibers embedded in a matrix of recycled high densitypolyethylene, the composite material is in the shape of a “Dog Bone”conforming to ASTM D 3039 Standard tor tensile testing;

FIG. 2 is a scanning electron microscopic image of the compositematerial at 80× magnification;

FIG. 3 is a scanning electron microscopic image of the compositematerial at 170× magnification along the grain;

FIG. 4 is a scanning electron microscopic, image of the compositematerial at 330× magnification cross the grain;

FIG. 5 is a scanning electron microscopic image of a fractured sample ofthe composite material after tensile testing at 430× magnification crossthe grain;

FIG. 6 is a scanning electros microscopic image of a fractured sample ofthe composite material after tensile testing at 600× magnification alongthe grain;

FIG. 7 is a graph of the Strain-Stress curve of composite sampleconforming to ASTM D 3039 tensile strength testing standard;

FIG. 8 is an image of a hybrid yarn comprising hemp fibers andpolylactic acid fibers for use in a pultrusion process;

FIG. 9 is a depiction of a braided hybrid yarn comprising hemp fibersand polylactic acid fibers;

FIG. 10A is a depiction of the braided hybrid yarn of FIG. 9 after it iscut open and laid flat;

FIG. 10B is a depiction of the cut open braided hybrid yarn of FIG. 10Aafter being consolidated;

FIG. 11 is a depiction of a pultrusion process using a hybrid yarn(hybrid yarn) and a melted polymer resin to produce a final composite;

FIG. 12 show various possible embodiments of articles comprising acomposite material of hemp fibers embedded in a recycled high densitypolyethylene matrix that may be formed by a pultrusion process;

FIG. 13 depicts a mat woven from hemp fibers;

FIG. 14 depicts a vacuum assisted resin transfer process for productionof a composite material comprising hemp fibers embedded in a matrix ofrecycle high density polyethylene;

FIG. 15 depicts a vacuum bag assisted resin transfer process forproduction of a composite material comprising hemp fibers embedded in amatrix of recycle high density polyethylene.

FIG. 16(a) is an SEM image showing the surface morphology of a compositecomprising recycled HDPE with untreated hemp fibers; FIG. 16(a) is anSEM image showing the surface morphology of a composite comprisingrecycled HDPE with 5% NaOH treated hemp fibers; FIG. 16(c) is an SEMimage of the fracture surface of a composite with 30% untreated hempfiber volume fraction showing fiber pull out from the rHDPE matrix; andFIG. 16(d) is an SEM image of the fracture surface of a composite with30% volume traction of NaOH treated hemp/rHDPE matrix;

FIG. 17 is a graph of the FTIR spectra for both untreated and treatedhemp fiber, the treated hemp fiber was immersed in 5% NaOH for 24 hours;

FIG. 18 are graphs of strain-stress curves of hemp fiber composites withdifferent fiber/matrix volume fractions, wherein the hemp fibercomposites comprise fibers treated with NaOH;

FIG. 19 are graphs of strain-stress curves of hemp fiber composites withdifferent fiber/matrix volume fractions, wherein the hemp fibercomposites comprise untreated hemp fibers;

FIG. 20 is a graph of the maximum tensile strength for the treatedhemp/rHDPE composite with 40% of fiber volume, the testing demonstratesan approximate three times improvement over the treated hemp compositeWith 20% of fiber volume fraction, the treated hemp/rHDPE composite with40% of fiber volume yielded a maximum strength of 60.2 MPa and a strainat maximum strength of 3.0;

FIG. 22 is a graph of the flexural stress of the treated hempfibers/rHDPE composites, as a function of the flexural strain;

FIG. 23 is a graph of the maximum flexural strength of variousembodiments of the composites comprising hemp fibers embedded in amatrix of rHDPE; and

FIG. 24 is a graph of the flexural moduli of the composites withdifferent fiber volume fractions.

DESCRIPTION OF EMBODIMENTS

The composite will be described more fully hereinafter by describingembodiments of the composite material, in which some, but not allembodiments of the composite materials are described. Indeed, thisinvention may be embodied in many different forms and should not beconstrued as limited to the embodiments set forth herein; rather, theseembodiments are provided so that this disclosure will satisfy applicablelegal requirements.

The invention is directed to composite materials. Embodiments of acomposite material comprise hemp fibers embedded in a matrix of highdensity polyethylene. The hemp fibers reinforce the matrix such that anarticle made from the composite has a greater tensile strength than anarticle made from the high density polyethylene alone. For example,embodiments of the composite material may have a tensile strength ofgreater than 40 MPa, further embodiments of the invention may have atensile strength of greater than 40 MPa, and still further embodimentsof the invention may have a tensile strength of greater than 60 MPa.Tire increase in tensile strength may be due to pretreatment of the hempfibers prior to incorporation into a matrix of high densitypolyethylene.

“Hemp fibers” or “hemp” are durable fibers that are harvested fromplants of the Cannabis genus, for example. The term “hemp” may also beused to identify the entire plant from which hemp fibers are derived.The hemp fibers are considered to be the most commercially valuable partof the hemp plant. Bast fibers give the plants its strength. The term“bast fibers” refers to the fibers that grow on the outside of the woodyinterior of the plant's stalk and under the bark. Examples of hempfibers are shown in FIG. 1. In the hemp plant, bast fibers grow to beapproximately 3 feet to 15 feet long and are used for industrialpurposes including paper, textiles, biodegradable plastics,construction, health food and fuel with modest commercial success.

Hemp is an important source of fibers because it is one of the fastergrowing biomasses. For example, hemp farming may produce up to 25 tonsof dry matter per hectare per year. The dry matter includes,approximately, one ton of hemp bast fiber per 3-4 tons of the dry rettedhemp straw.

Cannabis saliva L., subsp. sativa var. sativa is the variety of hempplant grown for industrial hemp production. Another variety, Canabissativa subsp. indica, has poor fiber quality and is primarily used forproduction of recreational and medicinal drugs.

The chemical composition of natural fibers consists essentially ofcellulose (microfiber of the cell wall), hemicelluloses. and lignin(biopolymer components of the cell wall). The outer surfaces of plantfiber contain waxes, fats, and pectin. The cellulose group is a highlycrystalline structure with theoretical Young's modulus of ˜130 GPa;therefore many natural fibers exhibit good mechanical properties. Inparticular, hemp, flax, and kenaf have remarkable mechanical properties,with a comparable specific strength but higher specific modulus thanE-glass fiber, as presented in Table 1.

TABLE 1 Typical Mechanical Properties of Cellulose Fiber vs. E-glassFiber Tensile Elastic Specific Specific Elongation Moisture 2011 DensityStrength Modulus Strength Modulus at failure Absorption Cost Material(g/cc) (MPa) (GPa) (σ/γ) (g/γ) (%) (%) ($/lb) E-glass 2.62 3400  731297  28 4.8 N/A 1.10 Hemp 1.4 550-900 70 393-643 50 1.6  6-12 0.30 Flax1.4  800-1500 60-80  571-1071 43-57 2.7-3.2  8-12 0.33 Ramie 1.5 500 44333 29 3.6-3.8  8-17 0.34 Kenaf 1.45 930 53 641 36 1.6 10-12 0.24 Coir1.25 220  6 176  5 1.5-4   8 0.20 Sisal 1.33 600-700 38 451-526 29 3-710-22 0.36 Jute 1.46 400-800 10-30 281-548  7-21 1.5 12-14 0.20

The composite materials comprise high density polyethylene. The highdensity polyethylene may be a recycled high density polyethylene(rHDPE). As used herein, “recycled high density polyethylene” is highdensity polyethylene that has been manufactured from used products orwaste materials comprising high density polyethylene. The used or wastematerial may be reprocessed to produce the recycled high densitypolyethylene. Typical reprocessing steps tor recycling high densitypolyethylene include, but are not limited to, cleaning the used or wastematerial, segregating and/or sorting of the plastic according to resincontent and/or color, shredding or chopping the plastic, and/or furthercleaning or processing the shredded plastic to remove any remainingcontaminants such as paper, glass or any other impurities. The cleanedand shredded plastic may then be melted and extruded in the form ofpellets or granules. The pellets or granules of recycled plastics maythen be reused to produce the composite materials. An embodiment of acomposite material comprising hemp fibers embedded in a matrix ofrecycled high density polyethylene is shown in FIG. 1. The compositematerial in FIG. 1 is in the form of a standard tensile strength “dogbone” conforming to the requirements of ASTM D 3039 testing. Tensiletesting indicates that the composite material comprising hemp fibersembedded in a matrix of recycled high density polyethylene has a tensilestrength of greater than 30 MPa.

The hemp fibers may be processed prior to being incorporated into thematrix of high density polyethylene. For example, the hemp fiber may bephysically processed such as, but not limited to, by washing, drying,drying under vacuum and heat, by cleaning, combing, carding, pulping,chopping, weaving, and spinning, for example, and/or chemical processedsuch as, but not limited to, by steam treatment, alkalization, dyeing,and/or other chemical treatment to provide any desired properties, forexample. In certain embodiments, the hemp fibers are cut or chopped. Forexample, the fibers may be cut to have an average length of about 0.5inches to 2 inches. The hemp fibers may be washed or rinsed withdistilled water, deionized water, aqueous solutions or organic solvents,for example. In certain embodiments, the hemp fibers may be washed withdistilled water and deionized water before and after chemical treatmentand thoroughly dried prior to incorporation into a composite. As usedherein, thoroughly dried means that the hemp fibers comprise less than1% water remaining on their surface by weight.

In further embodiments of the composite material, composite articles andmethods, the hemp fibers may be chemically treated with an alkalizationprocess. An alkalization process may clean the fibers and expose theacidic nature of the cellulose of the fibers. In addition, otherchemical treatments may be performed to modify the chemical propertiesof the fibers. For example, chemical modification may modify theproperties of the fibers to adhere more closely and strongly withpolymeric matrix. In summary, chemically treated hemp fibers may havedifferent properties than untreated hemp fibers. The chemical treatmentmay be performed with a NaOH solution, for example.

The chemical treatment process, such as, but not limited to, analkalization process, for hemp fiber has at least three functions.First, the alkalization process will remove or partially remove thenon-crystalline structures including pectin, wax, lignin andhemicelluloses from the fiber; therefore more cellulose structure willbe exposed for fiber/matrix adhesion. Second, the surface area of hempfiber may be increased after the treatment. Third, the hydroxyl (—OH)groups on the surface of the fiber may be significantly increased, whichwill provide more active sites for fiber/matrix interface bonding. Anyprocess that performs produces any of these results may be performed inembodiments of the methods of the producing hemp fiber/HDPE composites.

Other than NaOH, several chemical treatments could also increase thefiber/matrix interface adhesion, including silane, benzoylation, maltedcoupling agent, etc. For HDPE polymer, treatment withaminopropyltriethoxy silane may produce an increase in the interfacialadherence between the fibers and HDPE matrix. The organo-functionalgroup of silane will cross-link the HDPE backbone, while the silanolmolecules will react with the hydroxyl groups (OH⁻) of the hemp fibreand to form stable covalent links of Si—O-cellulose. The mechanism ofusing malted coupling agent is similar to the silane group.

Physical treatment of the hemp fibers may further include an orientationprocess such as carding. The strength of the fibers is in theirlongitudinal direction and the isotropic or anitropic nature of theproperties of an embodiment of the composite material may be designed bycontrolling the orientation of the fibers in the composite. In certainembodiments, the hemp fibers may have fibers predominantly in a lengthdirection with approximately a 20% cross directional orientation. Theorientation of the fibers will affect the properties of the resultantcomposite. The composite material comprises hemp fiber and high densitypolyethylene. The composite material may have any desired concentrationof hemp fibers in the matrix that produces the desired properties of thecomposite-material. In most embodiments, the composite material willhave a composition of hemp fibers between 10% and 60% volume fraction ofthe composite material. In further embodiments, the composite materialwill have a composition of hemp fibers between 2.5% and 55% volumefraction of the composite material; in still further embodiments, thecomposite material will have a composition of hemp fibers between 30%and 50% volume fraction of the composite material.

In one embodiment, the novel composite material may be used as buildingand infrastructure material or high-performance structural material. Inanother embodiment, the novel composite material may be used as acomposite material for wood flooring or to produce pallets. In a furtherembodiment, the novel composite material of the present invention may beused in a variety of application, including packaging, pharmaceuticalproduct packaging, building and infrastructure applications, bridgedecking, and in retaining walls to replace the conventional materials,especially the pressured treated lumbers and wood plastic composites.

The composite material is attractive for these applications and othersin the market due to its beneficial properties including, but notlimited to, high specific strength, insect and fungi resistance, highwater and moisture resistance, no violate chemical components orprocessing, and sustainability. As compared to conventional glass fiberreinforced composites, embodiments of the composite material comprisinghemp fibers is reinforced with strong natural fibers. The use ofmaterial fibers results in a reduction of the cost and energy consumedto produce the raw materials of the composite material. In addition, theHDPE matrix may be produced from recycled HDPE such as heavy containerbottles. The recycled material, which is less expensive than virgin HDPEand also reduces solid waste sent to landfills.

Embodiments include methods of producing composite materials comprisinghemp fibers embodied in a matrix of high density polyethylene.Embodiments of the methods include, but are not limited to, vacuumassisted resin transfer methods or pultrusion methods, for example.

Hemp Preparation EXAMPLE 1 Composite Material Production by CompressionMolding

The hemp fibers were prepared for incorporation into the compositematerial. Industrial grade hemp fibers were obtained from a commercialavailable source in Canada. The hemp fibers were approximately 1 inchlong and were prescreened to have an average aspect ratio in the rangeof 0.0015 to 0.003. The hemp fibers were then chemically processed sothe fibers would have improved interfacial strength between the fiberswith the HDPE when incorporated into the composite. The hemp fibers werechemically treated with an alkalization process. The alkalizationprocess included: 1) rinsing the fibers with distilled water, 2) thenthe cleaned fibers were fully submerged in a 20% sodium hydroxidesolution for 60 minutes at 25° C., and 3) rinsed and dried at 60° C. for24 hours.

Recycled High Density Polyethylene

HDPE bottles (labeled with the recycling symbol and a No. 2) werecollected, washed and shredded for reuse. The shredded HDPE was meltedand extruded through a C.W. Brahender 16-14-000 Extruder operated at 350RPMs to achieve a substantially homogeneous recycled high densitypolyethylene resin. The extruded material was shredded through GElaboratory mills to form pellets.

Producing the composite, recycled HDPE pellets were placed into analuminum mold in a hydraulic hot press (Craver Inc, 40 ton) with thetreated hemp fibers. The hemp fibers and the pellets uniformly placedwithin the mold in layers. The weight ratio of components was controlledat approximately 40% hemp fiber and approximately 60% HDPE. The fiberswere oriented predominantly in length direction with approximately 20%of cross directional orientation. After filling the mold, a compressionmolding process was used to form the composite material. The compressionmolding process was performed with a nominal compression force of 100 KNfor 30 minutes at a temperature of 300° C. The press was cooled withwater at a rate of 10° C./minute. After the process, the resultingcomposite material was removed from the mold.

The resulting composite material was cut into specimens for differenttesting experiments conforming to ASTM standard requirements. Scanningelectron microscopy image of the sample before, see FIGS. 2 to 4, andafter tensile testing, see FIGS. 5 and 6, were taken to analyze thesurface morphology and interface adhesion. All specimens were sputteredwith a layer of gold prior to SEM examination. At 200 micron scale, seeFIG. 2, the hemp fibers surface affinity and compatibility for polymercomposites has shown to be improved by surface treatment. Theinterfacial adhesion between the hemp fiber with polymer chain has beengreatly improved by the chemical processing of the fibers, which led tothe superior mechanical stability.

Tensile strength, elongation at break and Young's modulus weredetermined by the tensile test corresponding to ASTM D 3039 standard.The results indicated that the invented material has maximum strength at48 MPa and elongation of break at 3.58%.

EXAMPLE 2 Composite Material Production by Vacuum Assisted ResinTransfer Molding

In one embodiment, the method of forming a composite material includes avacuum assisted resin transfer molding method. Methods for vacuumassisted resin transfer molding (VARTM) include an infusion process. Inthe infusion process, a vacuum draws a melted resin into a one-sidedmold comprising fibers. A cover, or bag, is placed over the top of themold to form a vacuum-tight seal. Then the vacuum is applied within themold, thus drawing the resin into the mold to embed the fibers in resin.This method is also referred to as a vacuum infusion process (VIP)molding.

More specifically, vacuum infusion is a process used for molding fibercomposite articles, where distributed fibers are placed in the bottomportion of the mold. The fibers may be in the form of rovings, bands ofrovings, or mats. The mats may be woven mats made of single fibers orwoven mats made of fiber rovings. As used herein a “roving” is acollection of bundles of continuous filaments or fibers in untwistedstrands. A second mold portion, which is typically made of a resilientvacuum bag, is placed on top of the fibers. A vacuum, typically 80 to90% of the total vacuum, may then be applied to the mold cavity betweenthe bottom portion of the mold and the vacuum bag, the liquid polymer isdrawn in and fills the mold cavity thereby embedding the fibers in amatrix of the resin.

The delivery of the resin to the mold can be improved by using a flowenhancement layer or distribution media. The effect of the distributionmedia is to decrease filling time and improve the resin distribution.

Thus, embodiments of the method of producing a composite material maycomprise placing hemp fibers in a vacuum assisted resin transfer mold.The hemp fibers may be in any desired form and may be a blend of hempfibers and additional synthetic or natural fibers. The hemp fibers, forexample, may be in the form of bundles of fiber bands, bands of rovings,mats woven from of single hemp fibers or woven mats made of fiberrovings. The additional fibers may be incorporated in the yarn, rovingor mat or added separately from the hemp fibers, for example. The vacuumassisted resin transfer mold may then be covered with a vacuum bag. Thehemp fibers (and optional additional fibers) are thus sealed within themold cavity and the composite article is now ready to be produced byaddition of the resin.

The resin is prepared for introduction of the resin. In specificembodiments, the resin comprises a recycled high density polyethylene.The method may comprise heating recycled high density polyethylene intoa semi-liquid state above the glass transitional temperature for therecycled high density polyethylene. The heated resin may be drawn intothe vacuum assisted resin transfer mold by applying a vacuum to themold. The resin is drawn into the mold by the vacuum to replace theevacuated gases.

In one embodiment, hemp fibers are twisted into long threads. Thethreads are then woven into a mat of hemp fibers. The mat of hemp fibersmay have a similar geometric structure with fiber glass mat. Second,each layer of hemp fiber was layered on a vacuum bag. Then, recycledHDPE is preheated above the glass transitional temperature intosemi-liquid state. The composite could be formed into any desired shapebased on the form/shape of vacuum bag. In large scale production, thisprocess could also be easily achieved by using injection moldingprocess.

EXAMPLE 3 Composite Material Production by Pultrusion

Pultrusion is a continuous low pressured molding process using fiberreinforcements in resins matrices. Typically, the resins arethermosetting resins. The fiber reinforcements are formed into acontinuous form and are drawn through a resin bath or injected withresin into the mold. The fibers are drawn through and impregnated withthe liquid resin. The fiber is formed to the desired geometric shape andpulled into a heated steel die. Once inside the die, the resin cure isinitiated by controlling the composite at elevated temperatures. Thecomposite laminate solidifies in the shape of the die, as it iscontinuously “pulled” through the die by the Pultrusion machine.

The term pultrusion combines the words, “pull” and “extrusion”.Extrusion is the pushing of material, such as a billet of aluminum,through a shaped die. Whereas, pultrusion is the pulling of thematerial, such as fiberglass and resin, through a shaped die. A typicalpultrusion process starts with racks or creels holding rolls of fibermat or doffs of fiber roving. The raw fiber is pulled off the racks andguided through a resin bath or resin impregnation system. Resin can alsobe injected directly into the die in some pultrusion systems.

The raw resin may be combined with fillers, catalysts, and pigments. Thefiber reinforcement becomes fully impregnated (wetted-out) with theresin such that all the fiber filaments are saturated with the resinmixture. As the resin rich fiber exits the resin impregnation system,the un-cured composite material is pulled through a series of dies. Thedies arrange and organize the fiber into a desired shape, whilesqueezing out excess resin.

Once the resin impregnated fiber is organized and excess resin removed,the composite will pass through a heated steel die. The profile thatexits the heated die is a cured pultruded fiber reinforced polymer (FRP)composite. This FRP profile is pulled by a “gripper” system. Eithercaterpillar tracks or hydraulic clamps are used to pull the compositethrough the pultrusion die. At the end of this pultrusion machine, thepultruded profiles are cut to the specific length.

Embodiments of a method of producing a composite material may comprise apultrusion process wherein the fibers comprise hemp fibers. In suchembodiments, the resin in the pultrusion process may be high densitypolyethylene including recycled high density polyethylene. Embodimentsof the method include forming a yarn of hemp fibers or a hybrid yarnfrom hemp fibers and a polymeric fiber. The hemp fibers may be in anydesired form and may be a blend of hemp fibers and additional polymericfibers including synthetic or natural fibers. The hemp fibers, forexample, may be in the form of bundles of fiber bands, bands of rovings,mats woven from of single hemp fibers or woven mats made of fiberrovings. The additional fibers may be incorporated in the yarn, rovingor mat or added separately from the hemp fibers, for example. Thepolymeric fibers may comprise polyester or a polylactic acid fibers. Inembodiments of the composite material, composite article or the method,the ratio of hemp fibers to polymeric fibers is in the range of about10:1 to about 1:100. In further embodiments, the ratio of hemp fibers topolymeric fibers is in the range of 1:5 and 1:20.

Embodiments of the method may comprise braiding hemp fiber yarn or thehybrid yarn to form a braided yarn or a braided hybrid yarn. The yarn,braided or unbraided, may then be pultruded with a high densitypolyethylene resin. The high density polyethylene resin may producedfrom recycled high density polyethylene.

The yarns may be braided by any desired braiding method. For example,the braided hybrid yarn may be in a tubular shape, in certainembodiments, the method may comprise cutting the tubular braided hybridyarn open to form an open cut braid and consolidating the hemp andpolymeric fibers.

To scale up the hemp fiber composite production for load-bearingstructural application, we use the different manufacturing methods byincorporating modern textile techniques.

Braiding the hemp fiber with pultrusion techniques.

-   -   1) Twist hemp fiber with Polyester or polylactic acid (PLA)        fibers into yarn, keep the hemp fiber with PLA fiber ratio as        1:10 to form a hybrid yarn. Embodiments of the yarn are shown in        FIG. 8.    -   2) The hybrid yarn consisting of hemp with PLA fiber may be        braided by using braiding machine as shown in FIG. 9.    -   3) The braided fiber yarn may be cut open and consolidate as        shown in FIG. 10.    -   4) The open cut braid may be pulled through a pultruder. The        final product could be pultruded into any desired shapes with        the appropriately designed dies. The schematics of FIG. 11 show        the process.

In an embodiment of vacuum assisted resin transfer techniques to form acomposite material comprise hemp fiber embedded in a matrix of recyclehigh density polyethylene the following steps were performed.

-   -   Using 3D weaving techniques to wave hemp fiber into the mat        shape, which has the similar geometric structure with fiber        glass mat with much less weight, as the schematic shown in FIG.        12.    -   2) Laying each layer of hemp fiber on a vacuum bag. Then, we        preheat the recycled HDPE into semi-liquid state above the glass        transitional temperature, which will impregnate the layered hemp        fiber mats by a vacuum pump. The schematic show in FIGS. 13 and        14 depict an embodiment of the process. The composite could be        formed into desired shapes with designed vacuum bags.

EXAMPLE 4 Comparative Testing

Compression molding techniques were used to synthesize hemp fibercomposites with fiber volume fractions of 20%, 30% and 40% respectivelyfor both treated and untreated fibers. Scanning Electron Microscopy(SEM) and Fourier transform infrared spectroscopy (FTIR) were used toinvestigate the surface morphology of the fiber and the resultantcomposite. The tensile strength, secant modulus, flexural strength andflexural modulus of the composites having different volume fractioncompounds were analyzed.

Materials

Industrial hemp fibers were obtained from Hempline Inc. (Delaware,Ontario, Canada). The average density of Hemp fiber was 0.86 g/cm³ witha typical diameter of 22.5 μm and length of 25 mm. The moisture contentof the raw industrial hemp fiber was approximately 6%. The rHDPE pelletsused in this study were obtained from Customer Polymer Inc. (Charlotte,N.C., USA) which were recovered from detergent bottle applications,having an average bulk specific density of 0.98 g/cm³, a melt index (MI)of 0.45 g/10 min at 190° C., and a melting temperature range from 130°C. to 190° C.

Composite Manufacturing

For this study the Embodiments of the hemp fiber/rHDPE composites wereprepared using both treated and untreated hemp fibers. The treated hempfibers were prepared using an alkali solution, which contained a 5%concentration of sodium hydroxide (NaOH), prior to the fabrication ofthe polymeric composites. The hemp fibers were immersed in the NaOHsolution for 24 hours at 60° C. to allow complete saturation. Afterimmersion, the hemp fibers were washed with running distilled (DI) waterwith 1% of acetic acid to neutralize any remaining NaOH molecules. Thehemp fibers were then removed from the DI water when their pH levelranged from 6.8 to 7.2 using an Orion 2 Star PH meter. The hemp fiberswere then placed in a drying oven at 60° C. for 24 hours. The oven driedhemp fibers were then stored in desiccators prior to being used tomanufacture the polymeric composites.

The polymeric composite materials were fabricated by using both a C.W.Brabender 19.05 mm single-screw extruder and Carver hydraulic press.Initially, the pellets of the rHDPE were ground using a laboratorymiller manufactured by Arthur Thomas Co, Swedesboro, N.J. The groundedrHDPE powder was then processed into rHDPE films using the single-screwextruder. The extruder was operated at a temperature of 180° C. with anextruder rotational speed of 60 rpm. The films which were extruded had atypical thickness of 0.3 mm and were then cut into a 254 mm×254 mmsheets for use in the composite manufacturing process.

-   -   A compression molding technique using the Carver hydraulic press        was used to manufacture the hemp fiber composites with the rHDPE        films using a fabricated mold [1] having the dimensions of 254        mm×254 mm. Each composite sample was manufactured by sandwiching        a layer of manually distributed treated or untreated hemp fiber        in between two layers of rHDPE films at a temperature of 180° C.        under a constant pressure of 1.5 MPa for duration of 15 minutes.        The fibers were placed using a disoriented (random)        distribution. These various built up sandwiches were used to        fabricate the desired final composites. The weights of hemp        fiber and rHDPE layers were controlled to maintain a 20%, 30% or        40% fiber volume fraction. A summary of the composite materials        which were manufactured is presented in Table 2. The fiber        volume fraction V_(f) was determined by using fee following        Equations of [1] and [2]:

V_(f)=(W _(f)/ρ_(f))/(W _(m)/ρ_(m))+(W _(f)/ρ_(f))

V_(m)=1−V_(f)   [2]

Where V_(f) denotes the volume fraction of hemp fiber, W_(f) is theweight of hemp fiber sandwiched in the composite, and ρ_(f) is thedensity of hemp fiber, V_(m), W_(m), and ρ_(m) represent the volumefraction, weight, and the density of rHDPE matrix, respectively. Beforemanufacturing the composite, the weight of fiber and rHDPE for eachlayer was measured using a Denver Instrument bench-top scale. Thedensity of composite of each composition was measured by displacementmethods conforming to ASTM D 792-08¹⁷. The measured density of eachfabricated composite is presented in Table 3

Composite Characterization and Testing Scanning Electron MicroscopicAnalysis (SEM)

Surface morphology of the treated and untreated hemp fiber, fiberdistribution and the fiber/matrix interface were analyzed by using AJSM- 6764 SEM. The SEM specimens were selected from bulk samples of theheated and untreated fibers, and then coated with a thin layer of goldby using a Denton Desk IV sputtering instrument. The SEM instrument wasoperated at room temperature with 10 kV. The surface morphology of thetreated and untreated hemp fiber and the hemp fiber/matrix interface ofthe rHDPE composites were observed.

Fourier Transform Infrared Spectroscopy Measurement (FTIR)

Chemical compound of untreated and 5% NaOH treated hemp fiber wereanalyzed using a Perkin-Elmer 100 Spectrometer (Boston, Mass., USA). Atotal of 8 scans were taken for each sample between 650 cm⁻¹ to 4000cm⁻¹, with a resolution of 8 cm⁻¹. Each sample was prepared in filamentform.

Composite Mechanical Strength

Tensile and flexural testing were conducted using an Instron 5582constant rate of extension (CRT) universal testing machine in accordancewith ASTM D638¹⁸ and D790¹⁹ respectively, under the following testconditions of: i) a cross-head speed of 1.3 mm/min, ii) air temperature23° C., and iii) 65% relative humidity. For the tensile tests on thevarious composites manufactured using treated and untreated hemp fibers,the typical tensile stress—strain behavior including analyses of themaximum tensile strength, strain at maximum tensile strength, and thesecant modulus at 2 percent (%) strain are presented and reported. Forthe flexural tests on the various composites manufactured using treatedhemp fibers, the typical flexural stress-strain behavior includinganalyses of the maximum flexural strength, strain at maximum strength,and the flexural modulus at 1 and 3% strain of are presented andreported.

Results Surface Morphology Results

FIG. 16(a) is an SEM image showing the surface morphology of a compositecomprising recycled HDPE with untreated hemp fibers and FIG. 16(a) is anSEM image showing the surface morphology of a composite comprisingrecycled HDPE with 5% NaOH treated hemp fibers. A comparison of FIGS.16(a) and 16(b) reveals significant differences between the surfacemorphology of treated/untreated hemp fibers. As can be seen in FIG.16(a), the as-received fiber exhibited smooth non-cellulose structureboundary layers with wax/protein composition and surface impurities.FIG. 16(b) indicated that the alkylation process removed the weakboundary layer of non-cellulose structure, therefore, the surfaceroughness and surface area of the hemp fiber have been significantlyincreased, likely resulting in improved interfacial adhesion betweenfiber and rHDPE matrix. FIG. 16(c) shows the SEM image of the fracturesurface of a composite with 30% untreated hemp fiber volume fraction.Fiber pull-out may be observed in the resin rich regions of thecomposite. This could be attributed to the poor fiber/matrix interfacedue to the weak surface boundary observed in FIG. 16(a), suggesting thatthe failure mechanism in the untreated/rHDPE composite could haveresulted from de-bonding. On the other hand, FIG. 16(d) presents atypical SEM image of the fracture surface of a composite with 30% volumefraction of NaOH treated hemp/rHDPE matrix. Fiber breakage withoutpull-out from the matrix was often observed in many areas within thetest specimen as shown in FIG. 16(d). This may suggest that there isimproved fiber/matrix interface adhesive strength after alkalitreatment. Since more fibers break during testing rather than pull outof the matrix the strength of the overall composite is greater.

FTIR results of NaOH Treated Hemp Fiber

FIG. 17 is a graph of the FTIR spectra for both untreated and 5% NaOHtreated hemp fiber. The spectra show various transmission bands. Alter24 hours of NaOH treatment, the peak at 1000 cm⁻¹ (—OH group) issignificantly increased with associated hydroxyl group available forfiber/matrix interface bonding. The reaction of hydroxyl bonds with thecarboxyl group is given in the range 3200-3600 cm⁻¹. The peak in thisrange has increased after the 24 hour treatment. The similar increasesin intensity for both 1000 and 3200-3600 cm⁻¹ band in hemp fibers withNaOH treatment have also been reported in previous literature²⁰

Compared to untreated fiber, the peak at 1250 cm⁻¹ of treated hemp fiberis clearly removed. This peak belongs to the C-O stretching of acetylgroups of lignin. It appears that the lignin is completely removed fromthe hemp fiber surface after NaOH treatment. Also, the hemicellulosesgroup is partially removed from the fiber surface after the NaOHtreatment as is evident by the decreased carbonyl peak at 1600-1650 cm⁻¹in treated hemp fibers.

The peak at 1740-1750 cm⁻¹ in untreated hemp has also been removed afterthe NaOH treatment. The elimination of this peak was most likely due tothe removal of pectin and wax present on the untreated hemp fibers. Thepeaks observed at 1100 cm⁻¹ and 2850 cm⁻¹ in untreated fibers alsodisappeared after treatment. The disappearance of 1100 cm⁻¹ peak couldbe explained by the reaction of NaOH with a secondary alcoholic group,and the peak at 2850 cm⁻¹ disappeared after NaOH treatment probably dueto the removal of a methane group.

Tensile Strength

The tensile strength of the hemp fiber composites with rHDPE weredetermined from data obtained in accordance with ASTM D638. The tensiletests were conducted using the standard dog bone shaped test couponhaving manufactured dimensions of 12.7 mm in width, 63.5 mm in lengthand a thickness of 2.5 mm. Five (5) coupons were made from each testsample composite. Table 2 presents a summary of the composite materialswhich were evaluated which included composites manufactured with treatedand untreated fibers.

TABLE 2 Description of the various composite tensile test samplesPolymer Composite Hemp Fiber Polymer Fraction Designation Fraction (%)Matrix (%) rHDPE 0% of treated hemp rHDPE 100 20 uHemp/80rHDPE 20% ofuntreated hemp rHDPE 80 20 Hemp/80rHDPE 20% of treated hemp rHDPE 80 30uHemp/70rHDPE 30% of untreated hemp rHDPE 70 30 Hemp/70rHDPE 30% oftreated hemp rHDPE 70 40 uHemp/60rHDPE 40% of untreated hemp rHDPE 60 40Hemp/60rHDPE 40% of treated hemp rHDPE 60

Typical strain-stress curves of hemp fiber composites with differentfiber/matrix volume fraction are presented in FIG. 18 for treated fibercomposites and FIG. 19 for untreated fiber composites. It should benoted that the axial strains were calculated based on the displacementof the CRT's cross-head movement and the initial clamp spacing for eachtest specimen. A continual improvement in maximum tensile strength and areduction in strain at maximum strength were observed with the increasein hemp fiber volume fraction for the treated fiber composites. There isa significant improvement in the tensile stress-strain behavior of thetreated fiber composites (FIG. 18) compared to the untreated fibercomposites (FIG. 19) which may support the findings from the SEM andFTIR which suggests there is improved interfacial adhesion due to thefiber treatment. Overall, the hemp/rHDPE composites were well behavedwith regard to their initial stiffness and each had a distinct rapturefailure ranging from 3%-7% strain, as can be seen in FIG. 20, themaximum tensile strength for the treated hemp/rHDPE composite with 40%of fiber volume demonstrated an approximate three time improvement fromthe treated hemp composite with 20% of fiber volume fraction, yieldingan maximum strength of 60.2 MPa and a strain at maximum strength of 3.0.The tensile testing results of the treated hemp fiber with recycled HDPEmatrix exceeded the previous reported data regarding hemp fibercomposites manufactured with virgin Polylactic Acid (PLA) matrix.

Since these manufactured embodiments of the hemp fiber/rHDPE compositesare being considered for their potential use in the civil and buildingconstruction sector as possible structural elements there is an interestat understanding their low strain behavior. Due to the nonlinearbehavior of the embodiments of the hemp fiber/rHDPE composites, a secantmodulus at 2% strain was selected to evaluate the low strain behaviorand stiffness. FIG. 21 presents the secant modulus at 2% strain as afunction of hemp fiber volume fraction from 20% to 40% for both thetreated and untreated composites. There is an observed continuousimprovement in the composite stiffness with the increase in fiber volumefraction for the treated fiber composites. The greatest increase incomposite stiffness was observed for the 30% hemp fiber volume fractionhaving an elastic modulus of 1670 MPa as compared to the secant modulusof 556 MPa for the 20% hemp fiber volume fraction. The secant modulus at2% strain for the treated hemp fiber composite with rHDPE matrix with a40% fiber volume fraction was 2574 MPa. A summary of the tensileproperties of maximum strength, strain at maximum strength, and secantmodulus at 2% strain which were measured during this study with theircorresponding results for the hemp fiber composites are presented inTable 3.

TABLE 3 Summary of tensile test results for hemp fiber composites SecantMaximum Std. Modulus at Std. Density Strength Dev Strain 2% Strain DevComposite (g/cc) (MPa) (MPa) (%) (MPa) (MPa) rHDPE 0.98 19.1 0.6 17.9441 18.5 20 uHemp/80rHDPE 1.00 15.7 2.4 4.5 546 34.8 20 Hemp/80rHDPE0.95 18.6 2.1 7.0 556 56.3 30 uHemp/70rHDPE 1.03 27.4 4.9 5.4 682 98.430 Hemp/70rHDPE 0.93 45.7 5.7 3.7 1670 163.5 40 uHemp/60rHDPE 1.06 26.05.2 3.3 986 157.9 40 Hemp/60rHDPE 0.89 60.2 7.3 3.0 2574 257.2

Flexural Strength

Based on improved tensile strengths of the treated hemp fiberEmbodiments of the hemp fiber/rHDPE composites, flexural strengthtesting was conducted only on composites manufactured from treated hempfibers. The flexural strength, strain at maximum strength, and flexuralmodulus at 1% and 3% strain for these Embodiments of the hempfiber/rHDPE composites materials were tested on the CRT testing machinein accordance with ASTM D790. Each three point flexural bending test wasconducted using a rectangular test coupon having typical dimension of25.4 mm in width, 6.35 mm in thickness and 127 mm in length. Five (5)coupons were made from each test sample composite. The same treated hempfiber composites which were manufactured for the tensile tests were usedfor the flexural tests as described in Table 2.

FIG. 22 presents the flexural stress of the treated hemp/rHDPEcomposites as a function of the flexural strain. The flexural strainswere calculated based on the procedure provided in ASTM D790 using thedisplacement of the CRT's cross-head movement. It is interesting toobserve that as the fiber fraction increased there was a proportionalincrease in bending strength and stiffness. The most significantimprovement bending strength and stiffness was observed in the higher40% fiber fraction composite. It can be clearly seen that with anincrease in fiber volume fraction there is an increase in the maximumflexural strength as is presented in FIG. 23.

Based on the potential use of these manufactured embodiments of the hempfiber/rHDPE composites in the civil and building construction sector aspossible structural elements, there is a further need in understandingtheir low flexural strain behavior. Due to the more uniform behavior ofthe embodiments of the hemp fiber/rHDPE composites, the secant modulusat 1% and 3% strain were selected to evaluate the low flexural strainbehavior and stiffness. The flexural moduli of the composites withdifferent fiber volume fraction are presented in FIG. 24. The resultsindicate there is an increase in composite flexural moduli with theincrease in fiber volume fraction associated with a correspondingreduction in strain. However, the observed moduli reduce in stiffness asthe flexural strain increases. This behavior may be beneficial withregard to absorbing impact loadings that a building structure may incur.

A summary of the flexural strength properties including the maximumflexural strength, strain at maximum strength, and flexural modulus at1% and 3% flexural strain which were measured during this study withtheir corresponding results are presented in Table 4.

TABLE 4 Summary of flexural test results for hemp fiber compositesMaximum Strain at Flexural Flexural Flexural Std. Maximum Modulus atStd. Modulus at Std. Composite Strength Dev Strength 1% Strain Dev 3%Strain Dev Designation (MPa) (MPa) (%) (MPa) (MPa) (MPa) (MPa) rHDPE17.8 0.8 3.1 628 27.6 474 23.7 20 Hemp/80rHDPE 32.5 3.6 3.6 1598 177 960105.6 30 Hemp/70rHDPE 37.1 5.4 5.8 2015 282.1 1217 170.4 40 Hemp/60rHDPE44.6 8.0 6.0 2429 437.2 1485 265.8

The embodiments of the described method and composite are not limited tothe particular embodiments, method steps, and materials disclosed hereinas such formulations, process steps, and materials may vary somewhat.Moreover, the terminology employed herein is used for the purpose ofdescribing exemplary embodiments only and the terminology is notintended to be limiting since the scope of the various embodiments ofthe present invention will be limited only by the appended claims andequivalents thereof.

Therefore, while embodiments of the invention are described withreference to exemplary embodiments, those skilled in the art willunderstand that variations and modifications can be effected within thescope of the invention as defined in the appended claims. Accordingly,the scope of the various embodiments of the present invention should notbe limited to the above discussed embodiments, and should only bedefined by the following claims and all equivalents.

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1. A composite material, comprising: natural fibers consisting of hempfibers, wherein the natural fibers are embedded in a matrix of highdensity polyethylene, and wherein composite material has a high tensilestrength of greater than 60 MPa.
 2. The composite material of claim 1,wherein the high density polyethylene is recycled high densitypolyethylene.
 3. The composite material of claim 1, wherein the hempfibers have an aspect ratio from about 0.0015 to about 0.003.
 4. Thecomposite material of claim 3, wherein the hemp fibers have an averagelength of about 0.5 inches to 2 inches.
 5. The composite material ofclaim 1, wherein the composite material has a high density polyethylenecomposition between 30 wt. % and 60 wt. % of hemp fiber.
 6. Thecomposite material of claim 2, wherein the hemp fibers were chemicallytreated with an alkalization process.
 7. The composite material of claim2, wherein the hemp fibers have the properties of hemp fibers that werechemically treated with an alkalization process.
 8. The compositematerial of claim 2, wherein the hemp fibers have a fiber orientationare arranged predominantly in a length direction with approximately a20% cross directional orientation.